In electronic circuits, certain classes of signals often have a voltage or a current whose value encodes information. The information can be a measured value of a quantity, a number, etc.
Such signals are used within the circuit for processing the encoded information. Processing, however, attenuates the strength of such signals, which renders them less reliable for further processing.
Transmission gates are electronic circuits that are used to control when such signals are to be transmitted within the circuit. Transmission gates receive the signal as an input, and generate the restored signal as an output, when it is desired. In so doing, a transmission gate restores such signals to their original strength, so that they can be used for further processing.